formula.expertYour record

Fan Affinity Law: Airflow vs Speed

Also known as fan laws · VFD airflow change

Q2Q1=N2N1\frac{Q_2}{Q_1} = \frac{N_2}{N_1}
sig figs

Worked example: 8000 cfm at 900 rpm re-sheaved to 1080 rpm → 9600 cfm = 271841.7 L/min. Press Try an example to run it live, then adjust anything.

Enter your known values, leave one input blank, and solves for the missing one. Tap a variable’s symbol to see what it means, with a typical value. Change any unit and the answer follows.

Where Fan Affinity Law: Airflow vs Speed fits

Your answer feeds
Round Duct Air VelocityFan Brake HorsepowerAir Sensible Heat (1.08 Rule)Air Latent Heat (0.68 Rule)
In this toolkit

See also

Air Latent Heat (0.68 Rule): Both use air volume flow.Air Sensible Heat (1.08 Rule): Both use air volume flow.Air Total Heat (4.5 Rule): Both use air volume flow.Fan Brake Horsepower: Both use air volume flow.Round Duct Air Velocity: Both use air volume flow.Fan Affinity Law: Power vs Speed: Both use shaft speed.
5 more relatedFan Affinity Law: Static Pressure vs Speed: Both use shaft speed.Pump Affinity Law: Flow vs Speed: Both use shaft speed.Pump Affinity Law: Head vs Speed: Both use shaft speed.Pump Affinity Law: Power vs Speed: Both use shaft speed.Pump Specific Speed (Ns): Both use shaft speed.
Here the solver did the work. Could you?

Fan laws →

UniversityFluid Mechanics, HVAC & Refrigeration

Test your skills in the Exam Room: new numbers every attempt. Free lessons for students, no sign-up, just pure learning.

See your Report Card
Learning zone

Fan Affinity Law: Airflow vs Speed explained

N2N1Q1Q2

Air is a fluid like any other, so a fan wheel obeys the same first affinity law as a pump impeller: 20% more rpm buys 20% more cfm. A balancer measuring 8000 cfm at 900 rpm who needs design airflow speeds the wheel to 1080 rpm and reads 9600 cfm. On belt-driven units the adjustment is made not with a dial but with sheaves: swapping the motor pulley for one 20% larger raises fan rpm by 20%, which is why an air-balance report always records both sheave sizes.

The universal field mistake is treating this as free. Airflow is linear but brake horsepower is cubic, so the 20% re-sheave that fixes the airflow deficiency raises fan power by 73% and will happily trip an already fully loaded motor. Check the motor nameplate amps before you cut the belt, not after.

Fan Affinity Law: Airflow vs Speed formula

Q2Q1=N2N1\frac{Q_2}{Q_1} = \frac{N_2}{N_1}
Where
  • Q1Q_1= Airflow at speed 1 (L/min)
  • N1N_1= Fan speed 1 (rpm)
  • Q2Q_2= Airflow at speed 2 (L/min)
  • N2N_2= Fan speed 2 (rpm)
In this toolkit

The pump and fan affinity laws

7 formulasHow flow, head and power scale with impeller speed and diameter — the first, second and third power relationships behind every VFD retrofit.

Revised 16 October 2026
Cite this page

formula.expert. (2026, October 16). Fan Affinity Law: Airflow vs Speed Formula & Calculator. https://formula.expert/solvers/fan-affinity-flow-speed

formula.experthttps://formula.expert/solvers/fan-affinity-flow-speed